Complex and mixed atypical absences and tonic epileptic seizures of 12 patients with the Lennox-Gastaut syndrome with onset after age 6 were studied. By complexes seizures we mean the associations of two elementaries epileptics fenomena and by mixed that of three or more. Complex and mixed epileptic seizures already reported for patients with the early form of the syndrome were identified. The seizures reported, but not classified, in the literature were named by us as follows: gestural and mimic tonic-atonic automatic, tonic-gelastic, atypical absence with atonic cervical-cephalic seizures. New forms were described and classified. The following terminology was proposed for the convulsive seizures: procursive tonic-automatic, and procursive hemitonic-automatic. The non convulsive seizures - atypical absences - were described also in terms of critical EEG findings, and named as follows: automatic alimentary with palpebral myoclonic jerks; versive with palpebral myoclonic jerks; with palpebral myoclonic jerks and cervical and oculogyric atonic seizure; with intermittent cervical oculogyric atonic seizure and palpebral myoclonic jerks; with intermittent cervical atonic and oculogyric seizure; with palpebral myoclonic jerks and tonic seizure in cervical flexion. It was found that polimorfism of the complexes and mixed seizures is greatest for patients with the syndrome of early onset when compared with that with the syndrome of late onset.